1. Field of the Invention
The present invention relates to improvements in techniques for dewatering of slurries, particularly slurries resulting from xe2x80x9cdredge spoilsxe2x80x9d.
2. Description of the Related Art
Dredge spoil disposal presents a significant problem for dredging contractors. The dredged sludge is suspended in water in the form of a slurry. Such slurries may create serious recovery problems if discharged directly into holding lagoons or settling basins. Accordingly, there is a need to efficiently dewater and reclaim the solid materials from these slurries.
Hydraulic dredges are effective excavation devices for removal of a wide variety of sediments from natural or man-made waterways. The dredges may remove sediments which are classified as contaminated or hazardous sediments as well as non-hazardous sediments. The sediments may comprise debris such as sand, gravel, clays, silts, organic matter, or any combination thereof. Typically, the finest fraction of solids, including clays, silts, and organic matter, contributes the greatest volume and this fraction is also the most difficult to recover. All of the materials excavated from a waterway in the hydraulic dredging process must be removed to a disposal site. Traditionally, these sites include settling basins specifically engineered to accommodate the slow settling characteristics of the finest of the particulate matter. These xe2x80x9csludge lagoonsxe2x80x9d burden the immediate community in various ways. They occupy significant areas of land and are generally unpleasant neighbors because of the noxious odors emitted during the slow process of drying out the hydrated sediments.
In an attempt to solve the problems associated with such traditional settling basins, dredge slurry dewatering systems were introduced. These dewatering systems are used to separate dredge solids from water so that the solids may be recycled or disposed. The systems typically comprise screening assemblies, hydrocyclones, centrifuges, belt presses and clarifying vessels. Most of these systems operate as xe2x80x9cbatch treatmentxe2x80x9d processes, which can accept only defined quantities of dredge spoil at spaced intervals. The dredge spoil is first pumped into a holding vessel, then, while the dredge sits idle, the slurry is slowly pumped and treated by each apparatus in turn, until the holding vessel can be refilled. The processing rates of the individual equipment components of the dewatering system thus limit the throughput rate of the dredging operation. The inefficiencies associated with such batch systems serve to raise the costs of operation.
U.S. Pat. No. 5,656,174 describes a dewatering system that is capable of continuous operation. The system employs a series of filtration screens, and optional further separating devices such as hydrocyclones, followed by addition of a flocculating agent to aggregate the fine solids. The flocculated material is then filtered by passage over a tracking screen assembly having two screens supported on an A-shaped frame. Although relatively high system throughputs (e.g. 1200 US gallons per minute [gal/min] [0.073 m3sxe2x88x921]) can be achieved, the described system has a number of shortcomings. There is no means for adjusting the addition of the flocculating agent in response to variations in either the flow rate or the pulp density of the slurry. Thus, an optimal quantity of flocculating agent might not be consistently added. As the flocculated material is added to the A-shaped tracking screens, local settling of the floccules may occur, creating xe2x80x9cmud banksxe2x80x9d within the apparatus. Such mud banks result in uneven delivery of material to the screen, with consequent inefficient usage of the full surface area. In extreme cases, clogging may cause the screens to become inoperative.
The present invention provides a system and method for dewatering a continuous stream of slurry at high flow rates, with enhanced efficiency, economy and ease of operation. A stream of slurry is subject to continuous or continual measurement of flow rate and pulp density and the measurements are used to control the rate of addition of a flocculating agent to the slurry. The flocculated slurry is delivered to a dewatering unit where it is passed through a diffuser which evenly delivers it to a tracking screen for filtering solids from a liquid filtrate. The pulp density of the filtrate may also be measured and used to control addition of a flocculating agent. The tracking screen is adjusted to achieve an angle of repose suitable for the composition of the filter cake. The diffuser has a base having a horizontal section aspect ratio of from about 1:1 to about 4:1 and continuously changing vertically into an upper portion having a horizontal section with an aspect ratio of from about 40:1 to about 100:1. The ratio of the horizontal sectional areas at the upper portion with respect to the base is from about 1:1 to about 3:1. The dewatering unit of the invention may be adjustably mounted on a transport means for deployment of the high speed dewatering system and method at any convenient location.